The invention relates to a hydroformylation process. More specifically it relates to such a process wherein the amount of heavies in a catalyst recycle stream is controlled.
It is well known that aldehydes can be produced by reacting olefins with carbon monoxide and hydrogen in the presence of a metal-organophosphorus ligand complex catalyst, and that preferred processes involve continuous hydroformylation and recycling of a catalyst solution containing a metal-organophosphorus ligand complex catalyst wherein the metal is selected from Groups 8, 9, or 10. Rhodium is a preferred Group 9 metal. U.S. Pat. No. 4,148,830, U.S. Pat. No. 4,717,775, and U.S. Pat. No. 4,769,498 disclose examples of this process. The resulting aldehydes can be used to produce a host of products including alcohols, amines, and acids. It is common practice to employ a vaporizer following the reaction zone for the purpose of separating products from the catalyst.
It is known that hydroformylation catalysts comprising rhodium and organophosphite ligands are capable of very high reaction rates; see, “Rhodium Catalyzed Hydroformylation,” van Leeuwen, Claver, Kluwer Academic Pub. (2000). Such catalysts have industrial utility, as they can be used to increase production rates, or to efficiently hydroformylate internal and/or branched internal olefins, which react more slowly than linear alpha olefins. However, it is also known, e.g., from U.S. Pat. No. 4,774,361, that under some conditions these catalysts lose rhodium in liquid recycle hydroformylation processes. A continuous loss of rhodium can increase catalyst costs dramatically, as rhodium is prohibitively expensive.
Although the exact cause of rhodium loss is unclear, it has been hypothesized in U.S. Pat. No. 4,774,361 and elsewhere that the loss is exacerbated by the low concentration of carbon monoxide (CO) and high temperature environment of a typical product separation step. U.S. Pat. No. 6,500,991 describes a means of slowing the loss of rhodium in an organophosphite-promoted process by cooling the concentrated catalyst following product removal, and then adding CO to the concentrated stream. U.S. Pat. No. 6,500,991 also describes adding CO to a depressurization/flash vessel prior to the separation step. For either option, the total pressure in the separation zone is taught to be less than or equal to 1 bar. Thus, the process of U.S. Pat. No. 6,500,991 attempts to stabilize the catalyst before and after the separation zone without directly addressing losses that may occur during the harsh environment of the separation step.
U.S. Pat. No. 8,404,903 describes a means of removing aldehyde product at greater than atmospheric pressure while employing relatively moderate temperatures. However, that process offers no means to control the CO content beyond changing the condenser temperature of the separation zone. This means of control is limited to a narrow range of CO partial pressures and requires an expensive refrigeration unit to condition such a large flow of gases. At the maximum total pressure (100 psia) and mole percent CO (16%) described in U.S. Pat. No. 8,404,903, a maximum CO partial pressure of 16 psia is possible, although at this high pressure, the separation zone production rate is unacceptably low, even for removal of the relatively volatile C5 aldehyde. This is due to the fact that an acceptable balance of vaporizer temperature and recycle gas flow are required to achieve an acceptable product recovery rate and rate of rhodium loss. U.S. Pat. No. 8,404,903 mentions that the presence of CO in the recycle gas should be beneficial for stability of the phosphite ligand, but there is no mention of slowing or preventing rhodium loss.
In view of the deficiencies of the prior art, there remains a need for a means of separating high boiling aldehydes from a rhodium-organophosphite hydroformylation catalyst while reducing the loss of rhodium.